MEG is predominantly used for the manufacture of polyester fibres, polyethylene terephthalate (PET) and, to a lesser extent, in the cooling systems of motor vehicles where it serves as antifreeze.
MEG may be produced by the homogeneously catalysed conversion of EO to EG, directly catalysed by e.g. bicarbonate or metalate salts. Alternatively, MEG may be produced by a 2 step process, a ketal type process (catalysed by acids) or via ethylene carbonate (EC) catalysed by e.g. alkali or alkaline earth metal halide, zinc, tin, alkyl amine or quaternary ammonium or the like. In the case of the latter, the integrated EO/EG process is usually split up into four sections: EO reaction and CO2 removal plus EO recovery; Light ends (LE) removal and EO purification; EC/EG reaction and EG recovery; and MEG purification. In the EC/EG reaction section, EO is reacted with CO2 to EC in the presence of a homogeneous catalyst. EC and catalyst are provided to a hydrolysis section in which EC is hydrolysed to MEG in the presence of catalyst, and catalyst is separated from MEG for recycle to the EC/EG reaction section.
In U.S. Pat. No. 6,080,897, a process is shown in which a catalyst separation vessel separates MEG from catalyst by evaporation of MEG, and evaporated MEG is subsequently condensed again to provide a liquid MEG feed to a main MEG purification column for stripping, rectification and pasteurisation. In a process of this type, the MEG purification column operates at a higher pressure than the separation vessel and a pump or gravity force is required to transport the condensed MEG to the MEG purification column.
The disadvantage of this system is that the MEG stream needs to be evaporated twice incurring a high steam requirement. We have now found that a very significant steam saving and investment saving can be made by limiting the process to a single MEG stream evaporation, by omitting the MEG condensation and lowering the operating pressure of the MEG purification column which enables transport of vapours from the catalyst separation section to the MEG purification without the use of a mechanical transport device.